Membrane for the separation of liquid mixtures by pervaporation

ABSTRACT

A membrane for the separation of liquid mixtures by pervaporation has two surfaces, a first one representing a feed side and a second one representing a permeate side. The membrane is comprised of a swellable elastomeric polymer material and a reinforcing support embedded essentially parallel to the membrane surfaces in the polymer material. The reinforcing support is a woven mesh arranged asymmetrically so that the reinforcing support is adjacent to the permeate side.

BACKGROUND OF THE INVENTION

The invention relates to a membrane for the separation of liquidmixtures according to the pervaporation principle whereby the membraneconsists of a swellable elastomeric polymer or polymer blend (polymermaterial) in which is incorporated a membrane-stabilizing, respectivelyreinforcing support that is essentially parallel to the membranesurface, the membrane having a feed side and a permeate side.

A membrane of the aforementioned kind has been known from U.S. Pat. No.3,770,567. This known membrane is a so-called ion exchange membrane,i.e., with this known membrane the separation of the liquid mixture iseffected according to the electrodialysis principle. This known ionexchange membrane is provided with a support material so that it maywithstand greater pressures of the liquid mixture to be separated thanunsupported membranes. A complete inclusion of fabric fibers of thesupport into the polymer material is thereby an essential condition fora structural reinforcement of the membrane without any imperfectionswhereby the polymer material may be thinner locally than corresponds tothe fabric layer. The linear expansion due to swelling of the known ionexchange membrane is reduced from 14 to 17% (without support) to 3% whenthe fabric-type support is employed.

However, in membranes with which a separation of liquid mixturesaccording to the principle of pervaporation is carried out, the liquidmixture to be separated is fed to the membrane in a cross flow mode,whereas the permeate exiting from the membrane is removed in the form ofvapors either by applying a vacuum to the backside (permeate side) ofthe membrane or with the aid of a circulating carrier gas stream. Theseparation of the mixture components is achieved due to their differentmembrane permeability and may be performed even against the ratio ofvapor pressures, i.e., favoring the less volatile component. Themembrane permeability is composed of the sorption of the liquid feedmixture into the membrane, the diffusion through the membrane, and thedesorption at the backside of the membrane into the permeate vaporcompartment. The sorption, in this context, has the greatest impact onthe separation efficiency, i.e., the separation depends primarily on thepreferred sorption of one component of the mixture relative to the otheror others by the polymer membrane. The liquid sorption by the polymermembrane is associated with swelling of the membrane. The membraneswelling, which increases with the concentration of the preferentiallypermeating component in the feed mixture, is described by the so-calledsorption isotherms, which, in the simplest case, predict a linearincrease of the degree of swelling depending on the feed concentration.

A preferred application of the membrane separation by pervaporation isthe separation of aqueous-organic liquid mixtures. In particular,aqueous-organic liquid mixtures comprised of water containing arelatively minor amount of organic components, are widely encountered asorganically laden waste waters and as process effluents inbiotechnology. The separation of such mixtures with the selectiveremoval of the minor amounts of organics is performed by pervaporationvia membranes of elastomeric polymers (polymer material) such as, forexample, silicone rubber (polydimethylsiloxane), polyurethane, orpolyether-polyamide-copolymers. In contrast to evaporation,pervaporation allows the selective removal of organic compounds having aconsiderably higher boiling point than water from water.

A prerequisite for a favorable separation effect, expressed as theenrichment of the organic component in the permeate in connection withits depletion (removal) in the retentate, is the selective sorptioncapacity of the membrane polymer for the organic component of themixture, which is manifested by swelling. Elastomeric polymers have anespecially strong swelling tendency which, in itself, is a condition forthe desired separation effect. In summarizing the above, it may be saidthat with elastic membranes of a polymer, respectively, a polymer blend,employed according to the principle of pervaporation, a hindrance of theswelling would also result in a reduction of the sorption capacity ofthe membrane for the components to be pervaporated, resulting, inreturn, in a reduction of the separation effect of the membrane.

In the aforementioned ion exchange membrane according to the principleof electrodialysis (U.S. Pat. No. 3,770,567), the membranes are embodiedrelatively thick which is basically disadvantageous for a mixtureseparation according to the principle of electrodialysis. For thisreason, the known ion exchange membrane was provided with a fabricreinforcement for the aforementioned reasons of improved handlingproperties and the desired greater pressure stability in order tocounteract the membrane swelling. In the known ion exchange membrane theaforementioned measures reduced the swelling from between 14 to 17% to3% which is exactly contrary to the objective to be realized by thisinvention.

It is an object of the present invention to provide a membrane for theseparation of liquid mixtures which yields freely to the swellingpressure due to pervaporation without resulting in substantial changesto the surface area dimensions of the membrane, whereby the membrane isable to withstand high concentrations of swelling-inducing components inthe water as well as operational changes of the swelling stage, wherebyno foreign resistance counteracts the removal of the permeate vaporsfrom the membrane, and whereby the flux through the membrane, relativeto conventional membranes, is increased and the selectivity of themembrane is improved. Furthermore, the membrane shell may bemanufactured according to conventional manufacturing methods for knownmembranes so that they may be produced in a simple and inexpensive way.

SUMMARY OF THE INVENTION

The object of the present invention is solved by providing a support inthe form of a woven mesh for the separation of liquid mixtures accordingto the principle of pervaporation, which is, in the direction of passageof the mixture through the membrane, arranged asymmetrically such as tobe shifted to the permeate side.

The advantage of the inventive membrane is essentially that it isbasically a membrane formed of a homogenous polymer film in whichhowever a woven mesh is embedded which is asymmetrically shifted to thepermeate side. When performing a separation of a mixture according tothe principle of pervaporation, the feed side of the membrane is theside which faces the liquid mixture, respectively, the feed mixture,while the permeate side of the membrane is actually the side which isreinforced by the woven mesh and which is exposed to the vacuumgenerated by pumping off the permeate vapors at the permeate side of themembrane. Due to the swelling of the polymer of the inventive membrane,the flux and the selectivity, with respect to membranes of an identicalequivalent thickness having no woven mesh, are significantly increased(flux, respectively, flux density in g/m² ×h). The equivalent thicknessis defined as the membrane thickness which corresponds to the sameamount of polymer per surface area without the woven mesh.

According to a preferred embodiment of the invention the swelling willoccur essentially in the area of the feed side of the membrane whichborders the support, respectively, the woven mesh. Relative to the totalcross-section of the membrane, an anisotropic swelling stage of themembrane is observed. In this respect, the swelling at the feed sidereaches its maximum while the permeate side of the membrane isessentially free of any swelling. It is essential and advantageous forthe present invention that the woven mesh present at the permeate sideof the membrane, which is essentially free of any swelling, does notinfluence the swelling stage of the membrane. However, it preventsadvantageously a longitudinal expansion of the membrane which generallyoccurs due to the membrane swelling at the feed side.

In an advantageous embodiment of the invention the degree of swelling iscontinuously reduced from the feed side to the permeate side whereby, asmentioned before, a degree of swelling is preferably zero at thepermeate side of the membrane.

It has been shown that membranes having at least an equivalent thicknessbetween 5×10⁻² and 2×10⁻¹ mm, which furthermore may be manufacturedconventionally according to known casting processes for unreinforcedmembranes (without woven mesh), have advantageously high flux propertiesand a high selectivity.

The mesh-type support is preferably made from plastic fabrics, forexample, a polyester fabric. However, it is also possible to form themesh-type support in an advantageous manner from a metal mesh, wherebyit is also conceivable to provide the mesh-type support as a combinationof a plastic and a metal mesh.

In a preferred woven mesh of the membrane the thickness of the fibers isapproximately 2.5×10⁻² to 10⁻² mm and is comprised of a polyestermonofilament.

In general, the membrane may be formed in any desired suitable manner,i.e., preferably it may be in the form of a flat sheet membrane suitablefor incorporation into membrane modules whereby, with respect to itsproperties, it corresponds essentially to the properties of non-swollenpolymer films. However, it is also possible, according to anotheradvantageous embodiment, to provide the membrane as a tube or tubularmembrane and it may then be used in apparatus employing tubularmembranes of the non-swollen kind.

With membranes embodied in a tubular shape the outer side of themembrane or, depending on the application, the inner side of themembrane may be the permeate side, i.e., the respective selected side isasymmetrically provided, relative to the membrane cross-section, withthe support, respectively, the woven mesh.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with the aid of the only schematicdrawing and a plurality of diagrams with respect to one embodiment. Itis shown in:

FIG. 1 a view of the cross-section of the membrane and schematically thesize of the membrane swelling vector of the respective swelling;

FIG. 2 shows the flux of nitrobenzene and water through an inventivemembrane as a function of the feed concentration, whereby the mixture isfed to the feed side (support down) and to the permeate side (supportup);

FIG. 3 shows the flux of phenol through an inventive membrane as afunction of the feed concentration, whereby the mixture is fed to thefeed side (support down) and to the permeate side (support up) as wellas to a membrane without support; and

FIG. 4 shows a cross-sectional view of a membrane based on an electronmicroscope photograph, from which it can be taken that the mesh-typesupport is not in contact with the polymer forming the membrane.

DESCRIPTION OF PREFERRED EMBODIMENTS

The membrane 10 is comprised essentially of a polymer 11 into which asupport 14, in the form of a woven mesh, is asymmetrically embedded withrespect to the cross-section. The asymmetric arrangement of the support14 within the membrane, respectively, within the polymer 11 forming themembrane 10 is such that the support 14 is shifted toward the permeateside 18, compare FIG. 1. One of the membrane surfaces 12 represents thefeed side, i.e., the side to which the mixture to be separated is fed,while the other membrane surface 13 represents the permeate side 18where the permeate vapors corresponding to the mixture 16 passingthrough the membrane 10 in the direction of passage 15 is removed by avacuum.

The two arrows which are shown below the membrane extending in atransverse and longitudinal direction correspond to the swelling vectorsand represent the order of magnitude of the degree of swelling of thepolymer 11 at the membrane surface and in the transverse direction ofthe membrane. The represented membrane exhibits a maximum swelling ofthe polymer 11 at the feed side 17 and exhibits practically no swellingat the permeate side 18. The intermediate stages of the degree ofswelling of the membrane 10 are represented in FIG. 1 as the dottedprofile line 19. The woven mesh 14 which is arranged at the permeateside of the membrane 10 which is essentially not swollen does notinfluence the swelling stage of the membrane 10 in this area. However,it prevents a longitudinal expansion of the membrane usually resultingfrom the membrane swelling at the feed side which now may only occur inthe transverse direction, i.e., in the form of a thickening of themembrane. This is represented by the aforementioned swelling vectorswith respect to the linear expansion directions of the membrane 10 underpervaporation conditions.

For the manufacture of the membrane 10 with the asymmetrically embeddedwoven mesh 14 basically the same relatively simple methods used for themanufacture of homogeneous polymer films may be applied, i.e., themanufacture from casting solutions with volatile solvents or from apolymer melt. When comparing the effectiveness of membranes, thethickening of the membrane 10, resulting from the incorporation of thewoven mesh 14, relative to unreinforced polymer membranes is accountedfor by providing an equal amount of polymer per surface area unit.

Important characteristics of the membrane 10 may be taken from thefollowing examples. The polymer membrane is made from apolyether-polyamide-block copolymer, available under the trade namePebax 4033 and manufactured by Atochem, Paris, which has been proven tobe especially suitable for the pervaporation of high boiling organics,especially of phenols, from water. A preferred woven mesh is comprisedof a polyester monofilament having a fiber thickness of, for example,8.5×10⁻² mm, available under the trade name Estal and manufactured bySchweiz. Seidengazefabrik, Thal. Various membranes of an equivalentthickness between 7×10⁻² and 1.5×10⁻¹ mm corresponding to a structuralthickness between 2.2×10⁻¹ and 3.1×10⁻¹ mm were provided with anasymmetric support, respectively, woven mesh 14 embedded in the membrane10 and manufactured by a casting process. The pervaporation experimentswere carried out in an aqueous phenol solution of a concentration rangeof 100 to 50,000 ppm (5%) phenol at 50° C. The following membranes wereemployed.

I: Homogeneous polymer film without woven mesh having a thickness of1.5×10⁻¹ mm; this thickness corresponds to the equivalent thickness ofthe woven mesh membranes.

II: Polymer film with asymmetrically integrated polyester woven mesh ofa structural thickness 3.1×10⁻¹ mm, equivalent thickness 150-, in theinventive arrangement corresponding to FIG. 1.

III: Polymer film of the above description, however having the oppositearrangement, i.e. the woven mesh reinforcement is at the feed side.

EXAMPLE 1

Diluted solutions (100 to 1,000 ppm phenol in the feed). The followingexperimental results with respect to flux (flux density in g/m² h) andselectivity (enrichment factor for phenol) were observed for the threemembranes:

    ______________________________________                                        Feed    Membrane I  Membrane II Membrane III                                  Concentr.                                                                             Flux   Select.  Flux Select.                                                                              Flux  Select.                             ______________________________________                                         100 ppm                                                                              34     192      36   194    36    139                                 1000 ppm                                                                              42     143      46   152    42    120                                 ______________________________________                                    

It can be taken from this example that for diluted feed solutions andcorrespondingly low membrane swelling the unreinforced (I) and theinventively laterally reinforced membrane (II) have comparablepervaporation performance. However, since the reinforced membrane of anidentical polymer amount with respect to the surface area isrespectively thicker than the unreinforced comparative membrane thecomparable pervaporation performance may be considered an advantage ofthe inventive membrane. The asymmetric swelling behavior underpervaporation conditions presumed by the present invention is confirmedby the comparison of the two arrangements of the asymmetricallyreinforced membrane. When the woven mesh reinforced membrane side isfacing the feed (III), the selectivity under identical conditions issignificantly reduced with respect to the inventively applied membrane(II). The disadvantageous hindrance of the membrane swelling at the feedside is immediately recognized as the reduced sorption capacity of themembrane for the component of the mixture to be separated, for example,phenol. However, when the swelling capacity is maintained, as isintended with the present invention, then the woven mesh reinforcementis no hindrance.

EXAMPLE 2

Concentrated solutions (10,000 to 50,000 ppm phenol in the feed). Withthe three membranes the following test results were observed.

    ______________________________________                                        Feed    Membrane I  Membrane II Membrane III                                  Concentr.                                                                             Flux   Select.  Flux Select.                                                                              Flux  Select.                             ______________________________________                                        10000 ppm                                                                              92    54       137  53     105   52                                  20000 ppm                                                                             143    35       214  35     158   38                                  30000 ppm                                                                             195    26       300  26     216   26                                  40000 ppm                                                                             238    20       377  20     285   20                                  ______________________________________                                    

High concentrations of the organic component in the feed result in ahigh membrane swelling with a respective loss of mechanical stability.The stabilizing (reinforcing) influence of the woven mesh is expressedin the fact that under the conditions of the above example theunreinforced membrane (I) is not stable when the concentration of phenolin the feed is 50,000 ppm, while the reinforced membrane (II) retainedits integrity over the duration of the experiment. With respect to thepervaporation performance the test results show that the woven meshreinforcement in general increases the flux rate of the membrane withoutreducing the selectivity; at a sufficiently high membrane swelling theflux enhancement thus pertains equally to all components of the feedmixture, i.e., phenol and water. The comparison of the two arrangementsof the asymmetrically reinforced membrane (II and III) not only showsthe advantage with respect to the inventive configuration (II), but alsothat it represents a considerable improvement with respect to theunreinforced comparative membrane (I).

With respect to FIG. 4 it will be noticed that, after the manufacture ofthe membrane 10, the woven mesh-type support 14 is practically notcontacted by the polymer 11 of the membrane 10. A cavity system 20 isshown in the area of the woven mesh-type support 14 which is notcontacted by the polymer 11. This results in an increase of the surfacearea (desorption area) of the permeate side 18 and thus, by definition,an increase of the flux density of the membrane 10.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

What we claim is:
 1. A membrane for the separation of liquid mixtures bypervaporation, said membrane having two surfaces, a first one of saidsurfaces representing a feed side and a second one of said surfacesrepresenting a permeate side, said membrane being comprised of aswellable elastomeric polymer material and a reinforcing supportembedded essentially parallel to said membrane surfaces in said polymermaterial, said reinforcing support being a woven mesh arrangedasymmetrically so that said reinforcing support is adjacent to saidpermeate side.
 2. A membrane according to claim 1, wherein said polymermaterial swells essentially in the area defined between said feed sideand said reinforcing support.
 3. A membrane according to claim 2,wherein a degree of swelling of said polymer material continuouslydecreases from said feed side toward said permeate side.
 4. A membraneaccording to claim 3, wherein said degree of swelling approaches 0 atsaid permeate side.
 5. A membrane according to claim 1, wherein saidmembrane has at least an equivalent thickness of between 5×10⁻² to2×10⁻¹ mm.
 6. A membrane according to claim 1, wherein said reinforcingsupport is a polyester fabric.
 7. A membrane according to claim 1,wherein said reinforcing support is a metal mesh.
 8. A membraneaccording to claim 1, wherein said reinforcing support has a fiberthickness of 2.5×10⁻² to 2×10⁻²
 9. A membrane according to claim 1,wherein said membrane is a flat sheet membrane.
 10. A membrane accordingto claim 1, wherein said membrane is a tubular membrane.
 11. A membraneaccording to claim 10, wherein said permeate side is arranged on anouter side of said tubular membrane.
 12. A membrane according to claim10, wherein said permeate side is arranged on an inner side of saidtubular membrane.